Hydrogels are used in the biomedical field in wound care, dental care, burn care, and for controlled drug delivery systems. Hydrogels are also used in ophthalmic applications, such as contact lenses. Hydrogels prepared from hydrophilic polymers are effective in wound care because they are biocompatible and provide a sterile moist cover, and because they have a relatively high water content and properties that closely resembles living tissues. (See, Murat Sen and Esra Nazan Avcý, published online 16 Jun. 2005, in Wiley InterScience (www.interscience.wiley.com), DOI: 10.1002/jbm.a.30308.) Moreover, the ideal hydrogels would be soft, mechanically strong, yet pliable to be able to take up body contours.
The same properties also make hydrogels useful in cosmetic applications and they may be used as structuring agents, moisturizers, and/or anti-scar agents. Even though hydrogels used in cosmetic applications need not be sterile, the absence of polymerization byproducts would be a great advantage. In particular, hydrogels have the capacity to prevent scar formation and to control moisture. These properties make hydrogels desirable for rejuvenating skin, and in particular, facial skin.
Hydrogels are typically prepared by crosslinking of hydrophilic polymers. Generally, crosslinking of the polymer chains is produced by hydroxyl free radicals, generated by chemical crosslinkers and initiators, or by ionizing radiation.
In chemical crosslinking, the hydroxyl free radicals are generated by the addition of chemical crosslinkers and initiators to a solution of the polymers. Hydrogels prepared by chemical crosslinking generally have very low mechanical strength and hence must have a supporting material, e.g., a film, foam or gauze, to make them useful for most applications. Moreover, chemical crosslinking leaves unreacted initiators and crosslinkers and byproducts of the chemical reaction, which are either toxic, undesirable, or both. These contaminants require additional purification steps which are expensive and time consuming. Moreover, as a general rule, such hydrogels cannot be sterilized easily.
In radiation induced crosslinking, an aqueous solution of the hydrophilic polymers is irradiated with, for example, gamma rays. Advantageously, the ionizing radiation simultaneously crosslinks and sterilizes the hydrogel. Typically, conventional hydrogels must be packaged and shipped with the plastic molds or trays in which they were crosslinked and/or sterilized, which adds more cost to the production and shipment of the hydrogel, and which produces environmentally unfriendly waste.